The present disclosure relates to a semiconductor laser device.
In recent years, multilevel modulation optical communication that can increase the communication speed has been frequently used in an optical communication field. As a representative method of the multilevel modulation system, a coherent communication system using a phase-shift keying (PSK) system has been known. In the coherent communication, a local oscillator light source is required on a reception side, in addition to a signal light source on a transmission side.
In the coherent communication, because an optical phase is modulated according to a signal, reduced phase fluctuation is required for the signal light source and the local oscillator light source. As a characteristic value that is an index of the size of phase fluctuation, a spectral linewidth of laser emission light has been generally used. As the spectral linewidth of a light source used in the coherent communication is exemplified by using the index, the spectral linewidth is 500 kHz in 25-Gbaud quadrature phase-shift-keying (QPSK), and to achieve further multilevel modulation, the spectral linewidth of 300 kHz or less is desired. Further, the spectral linewidth changes with advancement of the multilevel modulation system, and in a modulation system having a high multilevel degree such as quadrature amplitude modulation (QAM), a narrower spectral linewidth is required.
It has been known that the spectral linewidth of the laser light emitted by a semiconductor laser device theoretically depends on optical power, threshold gain, linewidth-enhancement factor, and internal loss. In order to decrease the spectral linewidth of laser emission, it is important to design a resonator that decreases the threshold gain in an emission mode.
As a structure of a single-mode emission semiconductor laser device, a distributed reflector (DR) semiconductor laser device has been generalized in addition to a distributed feedback (DFB) semiconductor laser device, which has been frequently used in the past. In a simple term, the DR semiconductor laser device has a structure in which a DBR mirror is provided at the rear of the DFB semiconductor laser device, to reduce the threshold gain in the emission mode by reflection from the rear DBR mirror. The low threshold gain in the DR semiconductor laser device is preferable for a semiconductor laser device for coherent communication in which a narrow spectral linewidth is required (for example, see Japanese Patent Publication No. 5795126).
On the other hand, even in the case of the DR semiconductor laser device, it is preferable to introduce a phase shift structure for the single-mode emission, as in the DFB semiconductor laser device. The structure is referred to as “λ/4 shift” or “π shift”, which is for obtaining laser emission in the wavelength at the center of a stopband by inserting a phase shift of a length half the period of a diffraction grating into near the center of the semiconductor laser device. By introducing the phase shift structure, laser power is distributed so as to attenuate in an exponential manner in directions away from the phase shift position.
In the case of the DR semiconductor laser device, the exponential distribution exhibits sharp attenuation as a coupling coefficient in the DFB portion becomes large, as in the DFB semiconductor laser device. If the exponential distribution is sharp, light is strongly confined in the laser resonator, and thus the threshold gain decreases. Therefore, it is preferable that the coupling coefficient is high in order to narrow the spectral linewidth. However, in the DR semiconductor laser device, it becomes difficult to obtain the single-mode emission when the coupling coefficient is designed to be larger in order to narrow the linewidth.